Syringes for injecting gas or liquid samples into a chromatograph for analysis according to the prior art comprise an elongate glass barrel having a coaxial bore. A stainless steel cannula extends from one end of the barrel, the cannula being positioned partially within and in fluid communication with the barrel bore. A plunger extends from the opposite end of the barrel and is movable lengthwise through the bore toward and away from the cannula. The plunger is formed of stainless steel and has a tip made of polytetrafluoroethylene (PTFE). Glass and stainless steel are the preferred materials for the barrel, cannula, and plunger because they are robust, inert materials which will give long service and will not contaminate the samples with which they come into contact. PTFE is used as the plunger tip also because it is inert, but, unlike the stainless steel, it can more easily form a gas tight seal within the bore which enables the syringe to draw and inject gas samples into the chromatograph.
Although the PTFE tip provides an effective seal it is not without disadvantages. The disadvantages arise largely from PTFE's propensity for cold flow when subjected to stress. This effect manifests itself destructively during injection of gas or liquid into the chromatograph. To ensure accuracy of analysis and have minimum adverse effect on chromatograph performance the injection must take place rapidly so that the sample is injected into the chromatograph as quickly as possible. Injection durations measured in milliseconds for microliter syringes are desirable to meet accuracy requirements. To achieve short injection durations requires rapid movement of the plunger toward the cannula. To ensure that as much of the sample is injected as possible it is customary to have the PTFE tip contact the end of the cannula within the barrel bore at the end of its stroke. Large impact forces, as great as 4 kg, between the PTFE tip and the stainless steel cannula and the end of the glass barrel are the result. These relatively large impact forces impose large compression stresses on the PTFE tip, which, if unconstrained, would tend to cold flow in response to the stress and “mushroom” radially outwardly. However, even in its undeformed state, the tip engages the bore with an interference fit. The barrel bore therefore constrains deformation of the tip so the radial tip stress imposed by the impact further increases the hoop stresses in the barrel locally in the region where the cannula is attached to the barrel. This region of the barrel is inherently weak because residual stresses are concentrated there as a result of the manufacturing process which attaches the cannula to the barrel. Repeated injections subject the barrel to repeated increases and decreases in stress, causing the barrel to crack, sometimes after as few as 100 injections.
Another disadvantage attributable to the cold flow characteristics of PTFE is the lost motion between the stainless steel plunger and the PTFE tip. The tip is swaged to the plunger. Upon impact of the tip against the cannula and the end of the barrel, forces are imposed between the plunger and the tip which cause stresses in the PTFE. The PTFE cold flows in response and the swaged attachment between the tip and the plunger is degraded. Although the PTFE tip remains attached to the plunger it is able to move relatively to the plunger, resulting in what is commonly known as “lost motion”. Lost motion manifests itself during drawing and injection of the sample, because friction between the tip and the barrel prevents the tip from moving until the lost motion between the plunger and the tip is taken up. This adversely affects the accuracy of the analysis and the performance of the chromatograph because, when there is lost motion, motion of the plunger does not correspond exactly to motion of the tip, and it is the motion of the tip which draws and injects the sample, while it is the motion of the plunger which is controlled and measurable. Inaccuracies are introduced when the motion of the plunger does not correspond to motion of the tip.
Yet another disadvantage attributable to cold flow of the PTFE is the propensity of the tip to extrude into the cannula. Each time the tip contacts the cannula and the end of the barrel during an injection, compression stresses are induced in the tip. In response the tip tends to deform radially outwardly, but cannot because it is prevented by contact with the sidewall of the bore. However, the tip is free to expand into the bore of the cannula, there being no surface there to restrain tip expansion due to the compression stress. Expansion occurs with each injection and eventually a projection is formed which extends axially from the end of the tip. This projection grows with each injection, and may break off, either within the bore of the barrel or within the bore of the cannula. Either condition is unacceptable. If the projection is free within the bore of the barrel it adversely affects the accuracy of the draw and the injection. If the projection breaks off within the bore of the cannula it can block the cannula and prevent injection altogether. There is clearly a need for an improved plunger for chromatograph syringes which addresses the disadvantages of the prior art.